1. Field of the Invention
This invention relates generally to cell temperature measurement in a battery pack and, more particularly, to a method and system for determining the temperature of cells in a battery pack which does not require temperature sensors, but rather determines impedance of each cell under application of an AC voltage signal at a given frequency and uses the impedance along with a state of charge to determine the temperature of the cell.
2. Discussion of the Related Art
Electric vehicles and gasoline-electric or diesel-electric hybrid vehicles are rapidly gaining popularity in today's automotive marketplace. Electric and hybrid-electric vehicles offer several desirable features, such as reducing or eliminating emissions and petroleum-based fuel consumption at the consumer level, and potentially lower operating costs. A key subsystem of electric and hybrid-electric vehicles is the battery pack, which can represent a substantial proportion of the vehicle's cost. Battery packs in these vehicles typically consist of numerous interconnected cells, which are able to deliver a lot of power on demand. Maximizing battery pack performance and life are key considerations in the design and operation of electric and hybrid electric vehicles.
A typical electric vehicle battery pack includes one or more battery pack sections, with each section containing numerous cells in series to provide the required voltage. In order to optimize the performance and durability of the battery pack, it is important to monitor the temperature of the cells. It is not feasible or too costly to measure the temperature of each individual cell, so there are typically a few temperature sensors situated in scattered locations throughout the battery pack. These temperature sensors only measure surface temperature of cells, so data from the temperature sensors can only be used to determine an average battery pack temperature, and to identify any abnormally high or low temperatures that may occur.
While the use of traditional temperature sensors is well known and reasonably effective, problems can occur if any of the temperature sensors fail. Such failures could lead to inaccurate temperature readings, which may diminish battery pack performance. Failures can also require service visits for the vehicle, in order to replace the defective component. In addition, there is a practical limit to the number of temperature sensors which can be provided in a battery pack. Therefore, the temperature of each individual cell can only be estimated. Also, temperature sensor hardware adds to the cost of the overall system.
There is a need for a battery pack cell temperature measurement method which does not require physical temperature sensors. Such a method could reduce cost by eliminating the temperature sensors, improve reliability by avoiding replacement of failed temperature sensors, and improve battery pack performance and durability by providing temperature data for each cell rather than at just a few points in a battery pack.